Based on your example, it would appear that accumarray reserves the right to collect the column vector of elements in each bin in any order it chooses. If subs is sorted it apparently uses the order defined by subs (based on your example, this is not stated in the doc), otherwise it does something else. In light of this, I suggest that the Note should say: The order of the elements in the column vector passed to fun is unspecified; it is recommended that the output of fun should not depend on the order of the elements in its input data. Alternatively, the doc should clarify the guarantee of what will happen if subs is sorted.

Does the term "sorted" in the note mean "sorted by linear index as defined by sub2ind?" If so, this should be stated. We could run your example using [~,sortedOrder]=sortrows(subs) and get a different answer. In fact, when I did that I got C==B. Is that just coincidence?

Most importantly, I pulled this example right out of the doc page. Why would the doc page include an example that is explicitly identified by the Note as bad practice? Unless that was the point of the example, in which case the example should say "here's why you shouldn't do what we just told you not to do."

Now I might be able to go on and figure out the rest of the exampes ;) !